1. Field of the Invention
The present invention relates to a lens system for a video projector, and more particularly to a refractive lens system located in front of the cathode ray tube for projecting onto a screen a video image formed on a fluorescent face of the cathode ray tube.
2. Description of the Prior Art
In a color video projector of a three-tube type emitting red, green and blue light, respectively, the correction of chromatic aberration can be generally left out of consideration, since three identical projecting lens systems are located in front of each of the cathode ray tubes for forming three different monochromatic images so as to project them toward a screen for composing one complete color image thereon, each of the projecting lens systems is required to transmit only one monochromatic image. Strictly speaking, however, the monochromatic image is practically formed by the light of a plurality of different wavelengths within a narrow wavelength band in accordance with the spectral characteristic of the fluorescent material of individual cathode ray tubes. Especially, the images formed by the cathode ray tubes in charge of a green image and a blue image, respectively, are inadequate to be regarded as monochromatic images. Therefore, because of the above mentioned wavelength bands forming the "monochromatic image", the chromatic aberration of the video projector lens system cannot be neglected in a high definition video projector such as CRT for displaying characters utilized as a terminal of a computer.
The video projector lens system generally utilizes plastic lens elements to form aspheric surfaces thereon. But in the above mentioned high definition or middle definition video projector, a temperature dependent characteristic thereof also creates a significant problem. That is, a plastic lens element generally shows a greater degree of variation in refractive index depending on the change in temperature, in comparison with a glass lens element.